This invention relates to a bone replacement material and its use.
Attempts have always been made to fill bone gaps following bone injuries or surgical removal, since experience has shown that bone regeneration takes a long time and that an extremity's functioning is not restored until the bone has healed completely. It is hardly possible for large bone gaps to heal in such a way that the original function is restored. That is why attempts have been made for a very long time to use bone replacement materials, either as transplants or implants.
Unfortunately, the search for suitable materials has not been very effective. When using autologous material, i.e. from bone transplants from the same patient, the amount of material is limited since only certain parts of the locomotor system can be used to donate bone. Homologous material presents the problem of immune reactions--a problem which to date has not been solved. Although prepared homologous and heterologous bone material can be used to a limited extent for bone replacement, it is not fully accepted by the body and is not completely incorporated therein, as has been shown in experiments.
That is why attempts have been made to further treat animal bones and develop a bone replacement material capable of filling and bridging gaps. It has always been assumed that the bone's honeycomb structure should be maintained to the greatest possible extent.
All processes and all commercial materials, with the exception of the heterologous bone chips termed "cialit chips" or "Kiel chips", prepare animal bones or other collagens from animals in such a way that they are de-mineralized and lose their antigenicity through various chemical processes. The collagens obtained in this way are incorporated by the body and are also absorbable, but they are not suited for bearing biomechanical forces and do not serve as a stable supporting structure.
That is why attempts have been made again and again to develop materials with the properties of bone tissue, i.e. to provide support and to be absorbable. Various sintered tricalcium phosphates or apatites have been used primarily for this purpose. Attempts have also been made to facilitate the incorporation of total joint prostheses, for example, by structuring the surface of the implants. The incorporation of these prostheses can be compared to bone ingrowth in materials used for bone replacement implants. That is why similar demands are placed on the coating of a prosthesis, its structure and surface quality. To date, the problem of bone ingrowth and bone gap healing has not been solved in a satisfactory way. Bone replacement materials based on structured collagens, such as those proposed in DE-OS 28 54 490, do not exhibit a sufficient bone-building effect.
They indicate the structure the bone is to take, i.e. a trabecular structure, which only results in an inner and outer accumulation of reinforced and thickened trabeculae, but does not permit a normal bone architecture. In addition, due to de-mineralization, these materials no longer possess mechanical strength, thus explaining the lack in bone induction and excluding a supporting function. Sintered materials are hardly absorbable at all within a useful period of time.
The problems encountered in joint replacement are the same as those in bone replacement, plus the fact that the interface of a prosthesis is subjected to considerably more load than the interface of a bone replacement material. Prostheses, i.e. artificial joints, are generally anchored in the bone with a pin or stem (anchoring part) or are placed on the bone (see Journal of Bone and Joint Surgery, Vol. 21 (1939), pp. 269-288).
A common method of replacing a hip joint, for example, consists of inserting a full metal shaft into the bone marrow cavity and anchoring it there with a two-component plastic (bone cement) (J. Bone Joint Surg., Vol. 42B (1960), pp. 28-30). The known bone cements do not exhibit sufficient compatibility and biomechanical strength, however. During such procedures, it is generally necessary to enlarge the surface of the prosthetic stem. This is achieved, for example, by means of wave or saw tooth-like formations on the surface (see DE-PS 837 294). DE-OS 2 127 843 discloses a porous metal coating firmly connected with the base body of the same metal. Said coating is meant to enlarge the surface and permit bone ingrowth. Bone ingrowth only occurs under certain conditions with the known coatings. It is not possible to obtain reproducible results--transferrable to all patients--with such prosthetic surfaces. Thus, other factors play a role in ensuring bone ingrowth and anchoring for prognostic purposes.
In accordance with the invention, two main factors have been detected which induce the desired bone ingrowth and determine the morphology of a supporting or non-supporting bone structure and the related capability of bearing loads occurring without any difficulty. These are: 1. the morphology of the surface structure in the anchoring part and 2. the chemical composition (chemism) of its surface.
An experiment was conducted to determine the extent to which the individual bone-building cell, the osteoblast, and the osteoblast layer can be induced to build bone and specific trabeculae by means of different morphological and chemical structures and substances. A certain morphological structure in the anchoring part strongly induces bone formation by the bone-building cell, whereas a different morphological structure leads to the strongest formation of supporting trabeculae by the osteoblast layer. However, in order to achieve optimal static results, the required formations (topography) on the implant anchoring must be considered in the overall design. Moreover, the overall load and coordinated joint movement must also be taken into account with respect to prosthetic design which is decisive for the introduction of force.
Based on the above-mentioned findings and knowledge, in accordance with the invention, on bone induction and morphology as shaping criteria, four dimensions can be defined for the structuring of bone replacement materials. These shall be termed 1st to 4th order structures. On the basis of this definition, 1st order structure is the outer implant design, e.g. the shape of the prosthesis' anchoring part. The 2nd order structure represents the shaping of the surface (topography). Second order structure refers, for instance, to certain surface shapes such as a wave or saw tooth-like surface formation or a prosthetic stem with a step-like formation. The purpose of these 2nd order surface structures is to support mechanical anchoring and differentiate the anchoring surface with respect to load. In accordance with the present definition, 3rd order structure is the microstructure on the surface. This includes surface formations such as small spheres in the millimeter range. Finally, the 4th order structure refers to the ultrastructure with dimensions of approximately 20 .mu.m.
DE-OS 27 30 004 teaches an anchoring part for total bone prostheses, in particular pegs, the surface of which presents a number of projections connected in one piece with the base body and separated from one another by spaces. It is characterized by the fact that the space separating two adjacent projections presents at least one narrow spot located at the level between the surface and the highest points of the projections.
The features of this known anchoring part range from the prosthetic design (1st order structure) to microstructure (3rd order structure), the latter being defined by the projections on the surface. The purpose of the projections is to permit improved cross-linking of the bone tissue in the spaces between the projections, and thus a more resistant anchoring of the bone tissue, preferably without a binding substance.
The surface formation of this known anchoring part does present a disadvantage, however: It does not present an optimal morphological structure and has no shaping elements for the bone-building cell and the supporting cortical bone. Moreover, the surface structures are not absorbable, not even in part. Therefore, the adhesion of the bone cell is to the base body is not as good as in absorbable surfaces. In addition, the bioactive and chemotactic effect (bone induction) is not as great in non-absorbable surface coatings as in absorbable coatings, especially if active substances are admixed to the latter. Finally, the microstructure on the surface of the anchoring part known from DE-OS 27 30 004 cannot take on admixtures such as hemostatic agents, bone-building substances, antibiotics, vaso-active substances and bone-active hormones.
The greatest disadvantage is the fact that the surface structure does not permit the formation of continuous supporting arched constructions.
DE-OS 26 20 907 describes a prosthetic stem coating of absorbable ceramic materials (calcium phosphate-based) and non-absorbable plastics. When the ceramic is absorbed, a continuous porous plastic structure is formed with bioactivating ceramic remainders on the pores' inner surfaces.
A non-absorbable plastic matrix presents a disadvantage, however: The plastic is pulverized by the shearing forces occurring in the interface and the wear material cannot be absorbed. This results in inflammations and can cause the prosthesis to loosen. Another major drawback in the coating described in DE-OS 26 20 907 is the fact that the absorbable ceramic particles soak up the non-absorbable plastic, thus resulting in a further reduction in the coating's overall absorbing capacity. For this reason, the bone surrounding the prosthetic anchoring cannot grow deeply and quickly enough to the base body (support). This causes a reduction in strength.
A surface structuring of the anchoring part of a stem prosthesis is known from U.S. Pat. No. 3,855,638. A 100 to 1,000 .mu.m thick porous metal coating is applied to the substrate of the same metal. This coating consists primarily of spherical metal particles 50 to 150 .mu.m in size with 20 to 200 .mu.m large pores distributed in between. The dimensions and distribution of the spherical forming elements described in U.S. Pat. No. 3,855,638 as well as U.S. Pat. No. 4,206,516 are not very effective, however.
The forming element for the bone-building cell is less than 50 .mu.m and preferably 15-30 .mu.m in size. The forming element for the supporting trabeculae is between 580 and 1,000 .mu.m. The size distribution of the non-absorbable formative elements in the coating described in the U.S. patents does not permit a rapid and long-lasting bone anchoring. As indicated in the patents, the spaces permit an ingrowth of filamentous structures and even woven bones, but not an ingrowth and penetration of the mature, supporting bone structures required for larger spaces.
The problem of the invention is to create a bone replacement material suited for the coating and surface structuring of bone implants and for use as a shaped complete implant. It must be penetrated deeply and quickly by supporting bone structures, thus leading to stable implants capable of bearing loads.
The problem was solved due to the surprising finding that the elementary body layers present a network-like structure and leave nearly ideal morphological spaces in their cavity system. These spaces are filled with a coating mass consisting of absorbable fillers (based on strong calcium compounds) and an absorbable binding substance. This induces bone ingrowth and the formation of a supporting bone arch.